High-concentration active doping in semiconductors and semiconductor devices produced by such doping

ABSTRACT

In a method of forming a photonic device, a first silicon electrode is formed, and then a germanium active layer is formed on the first silicon electrode while including n-type dopant atoms in the germanium layer, during formation of the layer, to produce a background electrical dopant concentration that is greater than an intrinsic dopant concentration of germanium. A second silicon electrode is then formed on a surface of the germanium active layer. The formed germanium active layer is doped with additional dopant for supporting an electrically-pumped guided mode as a laser gain medium with an electrically-activated n-type electrical dopant concentration that is greater than the background dopant concentration to overcome electrical losses of the photonic device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/495,455, filed Jun. 10, 2011, the entirety of which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.N00421-03-9-002, awarded by the Naval Air Warfare Center-AircraftDivision, and under Contract No. FA9550-06-1-0470, awarded by the AirForce Office of Scientific Research. The Government has certain rightsin the invention.

BACKGROUND

This invention relates generally to techniques for electrically dopingsemiconducting materials, and more particularly relates tohigh-concentration doping of semiconducting materials in the fabricationof semiconductor devices.

High-concentration electrical doping of semiconducting materials isbecoming increasingly important for enabling advanced-performanceelectronic and optoelectronic devices. Conventionally, one or moreelectrical dopants can be added to a semiconducting material by, e.g.,in situ incorporation of a dopant during semiconducting material growth,by ion implantation into an existing semiconducting material, or bysolid- or vapor-phase diffusion of a dopant into an existingsemiconducting material, among a wide range of other doping methods.

Whatever doping technique is employed, the dopant species that isincorporated into a semiconducting material must be electricallyactivated. That is to say that the dopant species must be positioned atsites in the semiconductor material lattice such that free electricalcarriers, i.e., holes or electrons, are contributed to the semiconductorconductivity by the dopant species to alter the conductivity of thesemiconducting material in a desired manner.

But the concentration of dopant that is active in a semiconductingmaterial can be much less than the dopant concentration that is actuallyphysically present in the material. Generally, defects in asemiconducting material, e.g., damage that is generated by the dopingprocess itself, can limit the activation of dopants. In general,high-temperature annealing has been shown to both enhance dopantactivation and reduce lattice defects. But the temperature that isrequired for a very high degree of dopant activation by annealing is formany applications too aggressive for integration into advancedsemiconductor fabrication sequences with nanometric device features.High temperature annealing processes also can cause a degree of dopantdiffusion that is sufficiently high to actually drive the dopant speciesout of the semiconductor material.

As a result, it is found that for many semiconducting materials, thereis some limit of activated dopant concentration beyond which mostconventional doping processes fail. For example, ion implantationenables full control of dopant location, including directionality, butfor many materials causes severe lattice damage that in general resultsin a low fraction of activated dopant even when high a highconcentration of dopant is physically present. In situ doping duringmaterial growth produces relatively minimal lattice damage, but for manymaterials, high in situ doping can reduce or even halt material growthby, e.g., surface poisoning. The resulting upper limit for in situdoping concentration that can be accommodated during material growth mayactually be far below the solid-solubility of the dopant species in thesemiconductor material. Solid phase diffusion is limited by thediffusivity characteristics of a given semiconductor material, andvapor-phase diffusion processes typically require a temperature thatcannot be tolerated in nano-scale device fabrication with manymaterials.

For a wide range of important semiconducting materials,high-concentration active doping has therefore remained difficult, andis in general impossible in the context of conventional high-throughputsilicon-based fabrication processes and equipment.

SUMMARY OF THE INVENTION

There are herein provided methods for electrically doping semiconductingmaterials for achieving a high concentration of activated dopant in thesemiconducting materials. These methods can be employed in a wide rangeof applications, including, e.g., the production of photonic devices,and structures for producing such photonic devices, that enablehigh-performance opto-electronic device operation.

In one method for electrically doping a semiconducting material, a layerof the semiconducting material is formed having a layer thickness, whilein situ incorporating dopant atoms through the thickness of the layerduring formation of the layer. The formed layer has a first dopantconcentration of a selected dopant type. Then additional dopant atomsare ex situ incorporated through the thickness of the semiconductingmaterial layer, after formation of the layer, to produce through thelayer thickness a second dopant concentration that is of the selecteddopant type and that is greater than the first dopant concentration.

With this method of doping, there is provided a method of forming aphotonic device, for example, by forming a germanium active layer on asilicon substrate and in situ n-type doping the germanium active layerduring formation of the active layer. A reservoir of n-type dopant atomsis formed at the germanium active layer after formation of the activelayer. The dopant atoms are then diffused from the dopant atom reservoirthrough the germanium active layer.

In a further method of forming a photonic device, a first siliconelectrode is formed, and then a germanium active layer is formed on thefirst silicon electrode while including n-type dopant atoms in thegermanium layer, during formation of the layer, to produce a backgroundelectrical dopant concentration that is greater than an intrinsic dopantconcentration of germanium. A second silicon electrode is then formed ona surface of the germanium active layer. The formed germanium activelayer is doped with additional dopant for supporting anelectrically-pumped guided mode as a laser gain medium with anelectrically-activated n-type electrical dopant concentration that isgreater than the background dopant concentration to overcome electricallosses of the photonic device.

There is provided a structure for forming a photonic device, including asilicon substrate and an active layer of germanium disposed on thesilicon substrate, with the germanium active layer including an n-typedopant concentration of at least about 5×10¹⁸ cm⁻³. A stack of at leastone dopant reservoir layer is disposed on top of the germanium activelayer. Each such dopant reservoir layer in the stack consists of a leasta partial monolayer of phosphorus dopant atoms. A germaniumencapsulation layer is disposed between each dopant reservoir layer inthe stack.

There is further provided an electrically-pumped photonic device,including two silicon electrodes, with each electrode characterized byan electrical loss factor that contributes to an electrical loss totalfor the photonic device. An active layer of germanium is disposedbetween the two silicon electrodes for electrical pumping of the activelayer. The germanium active layer supports an electrically-pumped guidedmode as a laser gain medium with an electrically-activated n-typeelectrical dopant concentration that is greater than a background dopantconcentration characteristic of the active layer as-formed, to overcomethe electrical loss total for the photonic device.

With these methods and structures, there are enabled a wide range ofhigh-performance opto-electronic devices and systems having operationalparameters that previously could not be attained. Further features andadvantages will be apparent from the following description andaccompanying figures, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of the processing conducted in atwo-step in situ-ex situ process for high-concentration activated dopingof a semiconducting material;

FIGS. 2A, 2B, and 2C are plots of the conduction and valence bands atthe band gap of bulk intrinsic Ge, tensile-strained intrinsic Ge, andtensile strained n+ doped Ge, respectively;

FIGS. 3A-3G are schematic cross-sectional views of an example two-stepin situ-ex situ doping process for high concentration activated dopingof a Ge active layer, including an ex situ doping process employingdelta dopant reservoir layers;

FIG. 4 is a plot of gas flow as a function of time for the two-stepdoping process depicted in FIGS. 3A-3F;

FIGS. 5A-5B are schematic cross-sectional views of an example ex situdoping process including ion implantation, to be employed subsequent toan in situ doping process for high-concentration activated doping of Ge;

FIG. 6A is a schematic perspective view of an example design of the mainbody of a vertical-injection, electrically-pumped Ge laser that can beproduced with the two step doping process of FIG. 1;

FIGS. 7A-7K are schematic cross sectional views of an examplefabrication sequence for producing the main laser body of FIG. 6A;

FIG. 8 is a plot of P dopant concentration as a function of depth for aGe active layer and delta doping layers as in FIG. 3F;

FIG. 9 is a plot of P dopant concentration as a function of depth for aGe active layer that is in situ doped and includes delta doping layersas in FIG. 3F, subsequent to an ex situ dopant diffusion step;

FIG. 10 is a plot of intrinsic and extrinsic diffusivity of P dopant inGe as a function of temperature;

FIG. 11 is a plot of P dopant concentration as a function of depth for aGe active layer having no background doping;

FIG. 12 is a plot of photoluminescence intensity as a function ofwavelength for a Ge active layer that is in situ doped and includesdelta doping layers as in FIG. 3F, subsequent to an ex situ dopantdiffusion step;

FIG. 13 is a plot of measured photoluminescence intensity as a functionof wavelength for a phosphorus-implanted Ge active layer for variousimplantation annealing conditions;

FIG. 14 is a plot of photoluminescence intensity as a function ofwavelength for a phosphorus-arsenic co-ion implantation into Ge activelayers for various implantation annealing conditions;

FIG. 15 is a plot of photoluminescence intensity as a function ofwavelength for a phosphorus-antimony co-ion implantation combinationinto Ge active layers for various implantation annealing conditions;

FIG. 16 is a diagram and schematic view of a vertical-injection Ge laserbody and a test set-up employed for experimentally measuring lasingemission from the laser body;

FIGS. 17A-17B are plots of measured laser intensity as a function ofwavelength for the laser of FIG. 16, below the laser's lasing threshold,and above the laser's lasing threshold, respectively, and

FIG. 18 is a plot of the measured laser emission power as a function ofcurrent density for the laser of FIG. 16.

DETAILED DESCRIPTION

Referring to FIG. 1, in a process 10 for doping a semiconductingmaterial, a first in situ doping step 11 is conducted. The term “in situdoping” refers herein to a process of incorporating dopant atoms into asemiconducting material during formation of the semiconducting material,e.g., during growth of a layer of the material. Subsequently, there isconducted a second, ex situ doping step 23 that is carried out on thesemiconducting material after its formation. The term “ex situ doping”therefore refers herein to a process of incorporating dopant atoms intoa semiconducting material, e.g., a layer of semiconducting material,which has been previously formed. As explained in detail below, it isdiscovered that the in situ and ex situ doping processes operatesynergistically to achieve a level of activated dopant concentrationthat is not achievable with either doping step alone, while preservingthe electrical and mechanical integrity of the semiconducting material.

The semiconducting material to be doped can be formed on a substrate 12,as shown in FIG. 1, which is provided with any desired composition andstructure. The substrate can be an electrically doped material or anintrinsic material, and can be formed of the semiconducting materialitself or another desired composition. The semiconducting material canbe formed on one or more layers that have been previously disposed onthe substrate, and such layers can be homogeneous or heterogeneous withthe underlying substrate and the semiconductor material to be formed.

As shown in FIG. 1, during the in situ doping step 11, there is providedsuch a substrate 12 to which one or more species 14 for forming thesemiconducting material are directed 16. The semiconducting materialformation process can be conducted as a vapor-phase process,liquid-phase process, or other suitable technique. Examples ofwell-suited vapor-phase material growth techniques include ChemicalVapor Deposition (CVD), Atomic Layer Deposition (ALD), Molecular BeamEpitaxy (MBE), and sputter deposition, among a wide range of suitableprocesses.

During the formation of the semiconducting material, one or more species18 for incorporating dopant atoms into the semiconducting material aredirected 20 to the substrate 12 along with the species for forming thesemiconducting material. As a result, as the semiconducting material isformed, dopant atoms 21 and semiconducting material atoms 22 areproduced on the substrate 12, such that dopant atoms are incorporatedinto the semiconducting material lattice through its thickness as, e.g.,a layer of the material is formed.

The in situ doping step is conducted under selected conditions, asexplained in detail below, that produce a desired concentration ofdopant atoms in the semiconductor material as-grown, herein termed thebackground doping level or background electrical carrier concentration.In general, this in situ-produced background doping level can beselected ideally for a given application. For any application, thisbackground doping level provides an electrical carrier concentrationthat is greater than the electrical carrier concentration of theintrinsic semiconducting material at a corresponding diffusiontemperature; that is, the carrier concentration provided by thebackground doping is greater than the intrinsic carrier concentration ofthe semiconducting material.

With a desired in situ background doping step complete, then, as shownin FIG. 1, the ex situ doping step 23 is conducted to furtherelectrically dope the semiconducting material, now from an externalsource. In this step, one or more species 24 are directed 26 to theformed semiconducting material 28 with its background dopingconcentration, to add additional dopant atoms to the background dopingconcentration. For example, a solid layer 29 of dopant atoms can beformed on the surface of the semiconducting material. Alternatively,dopant atoms can be introduced into the semiconducting material surfaceto some selected depth within the semiconducting material. The dopantspecies 24 can be the same as that employed in the in situ doping step,or can be different, and can include multiple dopant species as-desired.The dopant species in this ex situ dopant step are therefore provided asa dopant reservoir either at the surface of the material or within thematerial itself. Then the dopant from the reservoir is diffused 30through the bulk of the material by, e.g., a suitable thermal process asshown in FIG. 1.

This thermal process 30 causes the dopant species to diffuse through thesemiconducting material and thereby activates the dopant species throughthe bulk of the material. Thermal annealing or other process can beemployed here to a degree desired for a given application, to enablecontribution of holes or electrons by the dopant atoms to thesemiconducting material from the dopant reservoir. As explained indetail below, the parameters of such a dopant activation treatment mustbe considered within the constraints of a given fabrication sequence.

This ex situ doping process of dopant reservoir formation 23 and thendiffusion 30 can be carried out employing, e.g., a solid source ofdopant, a vapor source of dopant, or a liquid source of dopant, andfurther can employ ion implantation, plasma doping, or other selectedprocess, that provides dopant species which can diffuse through thesemiconducting material layer.

With this combination of in situ and ex situ doping steps, thebackground carrier concentration that is produced in a semiconductingmaterial by the in situ doping step is found to enhance the ex situdopant diffusion process while minimizing damage to the semiconductingmaterial. As a result, a high-concentration of activated dopant, with anelectrical carrier concentration that is inaccessible by the in situstep alone or by the ex situ step alone, can be incorporated into thesemiconducting material. The only physical limit to the achievabledoping level may be dopant precipitation under some conditions, but ingeneral, because the process is not in equilibrium, the solid solubilityof a dopant in the semiconducting material can be surpassed.

Specifically, the dopant atoms that are incorporated through the bulk ofthe semiconducting material by the in situ doping step operate tosignificantly enhance the diffusivity that is characteristic of thesemiconducting material. The diffusion of dopant atoms into thesemiconducting material during the ex situ doping step is therebyaccordingly significantly enhanced, and can overcome many competingprocesses that tend to reduce the ultimate doping concentration that canbe achieved. The diffusion of dopant atoms during the ex situ dopingstep is further enhanced by the preservation of the integrity of thesemiconducting material lattice during the in situ doping step.Relatively minimal crystal lattice damage occurs during the in situdoping step, and the resulting high degree of lattice integrity aidsdopant diffusion during the ex situ doping step. If the ex situ dopingprocess is one that also minimizes lattice damage and the incorporationof lattice defects, then dopant diffusion during the ex situ doping stepcan be further enhanced and a very high concentration of doping can beachieved.

One example semiconducting material for which this high-concentrationdoping process is particularly desirable is germanium. Germanium (Ge) isintrinsically an indirect bandgap semiconducting material but can beengineered as a pseudo-direct bandgap material. As shown in FIG. 2A,intrinsic bulk Ge is characterized by a direct energy band gap of 0.8eV, which corresponds to a wavelength of 1550 nm, the most widely-usedwavelength in optical communication. The energy difference between thisdirect band gap, at the Γvalley of the conduction band, and the smaller,indirect band gap, at the L valley of the conduction band, is about0.136 eV. To enable direct band gap transitions at the desiredwavelength of 1550 nm, energies of these conduction band valleys must bealigned.

It is found that when intrinsic Ge is subjected to mechanical tensilestrain, the Ge energy bands are correspondingly shifted, as shown inFIG. 2B. Under tensile strain, the energy difference between the Γvalleyof the conduction band and the L valley of the conduction band isreduced to about 0.11 eV, under tensile strain of between about 0.1% andabout 0.3%. Such tensile strain can be imposed by, e.g., growth of Ge ona substrate material, e.g., silicon, that is characterized by a crystalstructure that is correspondingly mismatched with that the Ge. Examplesof such methods are described in “Method of Forming Ge Photodetectors,”U.S. Pat. No. 6,946,318, issued Sep. 20, 2005, the entirety of which ishereby incorporated by reference. With the imposition of a condition of0.25% tensile strain, then as shown in FIG. 2C, an n⁺ dopingconcentration that provides about 4×10¹⁹ cm⁻³ electrons can fill the Lvalley to enable the desired 0.76 eV band gap for 1550 nm wavelengthtransitions.

Conventional in-situ n-type doping of Ge by CVD epitaxial processing isfound to produce an active carrier concentration of only about 10¹⁹cm⁻³, due to a delicate balance between dopant incorporation and dopantout-diffusion during Ge growth. Epitaxial growth at relatively lowertemperatures is understood to enable an increase in dopantincorporation, but in general can result in such a poor quality Ge layerthat only a small fraction of the incorporated dopant is activated.Molecular Beam Epitaxy (MBE) has been shown to enable activated n-typedopant concentrations of about 10²⁰ cm⁻³, but MBE is an impracticalprocess for many fabrication sequences in that conventional CMOSfabrication processing does not employ MBE. As a result, conventionalsilicon-based devices and structures cannot be monolithically integratedwith MBE materials. Ion implantation has been demonstrated to achieve ahigh concentration of activate dopant in Ge after rapid thermalannealing, but at a high implant dose the resulting implant damage isdifficult to remove and increases the optical loss in optical devicesformed of such implanted Ge. Long-duration furnace annealing is shown toremove such implantation damage but due to out-diffusion of dopantspecies during the anneal, results in significantly reduced dopingconcentration.

The two-step, in situ-ex situ doping process overcomes these limitationsto achieve a Ge doping level that enables practical implementation of Geas a laser material for optoelectronic systems. Considering phosphorus(P) doping of Ge, it is found that the in-situ doping step can produce acarrier concentration of 1×10¹⁹ cm⁻³ with P doping, and that diffusionof phosphorus from a dopant reservoir during the ex situ doping step isin the extrinsic doping region with a quadratic dependence on carrierconcentration.

In analyzing the two-step in-situ-ex situ doping method for phosphorusdoping of Ge, it is found that theoretical calculations predict thatbecause the vacancy formation energy in Ge of 2 eV is significantlylower than that in Si, at 3.5 eV, vacancies play a more important rolein Ge than in Si in dopant diffusion. Ge self-diffuses by a vacancymechanism in which vacancies act as acceptors. N-type dopants, such asP, As, and Sb, have higher diffusivity in Ge than the diffusivity of Geself-diffusion. This fact reveals that an attractive interaction betweenn-type dopants and Ge vacancies results in the formation ofdopant-vacancy pairs (DV⁻) during dopant diffusion through Ge. It istherefore, reasonable to impose a vacancy mechanism on the modeling of Pdopant diffusion in Ge.

The charge state of a lattice vacancy in Ge is assumed to be doublynegative. The extrinsic, doubly charged Ge diffusivity, D_((DV)) ⁻ , cantherefore be considered to depend quadratically on electrical carrierconcentration, and can be expressed as:

$\begin{matrix}{{D_{{({DV})}^{-}} = {D_{0}( \frac{n}{n_{i}} )}^{2}},} & (1)\end{matrix}$where D₀ is the intrinsic diffusivity of Ge; n is the equilibriumcarrier concentration in a given Ge sample and n_(i) is the intrinsiccarrier concentration in tensile-strained Ge at the dopant diffusiontemperature of the ex situ doping step.

Given a 0.25% tensile strain in the Ge, then the resulting reduction inthe bandgap and the splitting of the valence bands causes the intrinsiccarrier concentration, n_(i), in tensile-strained Ge to be given as:

$\begin{matrix}{{n_{i} = {\sqrt{N_{c}( {N_{lh} + {N_{hh}{\exp( \frac{{- \Delta}\; E}{k\; T} )}}} )}{\exp( {- \frac{E_{g}}{2{kT}}} )}}},} & (2)\end{matrix}$where N_(c) is the effective density of states of electrons in theconduction band, N_(lh) is the effective density of states of lightholes in the valence band, N_(hh) is the effective density of states ofheavy holes in the valence band, and ΔE is the splitting energy of lightand heavy holes at the Γ valley.

With this intrinsic concentration thusly specified, then phosphorusdopant diffusion in Ge during the ex situ doping step can be describedby Fick's second law as:

$\begin{matrix}{\frac{\partial{n(x)}}{\partial t} = {\frac{\partial}{\partial x}{( {{D_{{({DV})}^{-}}(n)}\frac{\partial n}{\partial x}} ).}}} & (3)\end{matrix}$

With this analysis, it is shown that the extrinsic diffusivity of insitu-doped Ge is significantly increased over that of intrinsic Gebecause of the quadratic dependence of the diffusivity on carrierconcentration. The phosphorus diffusion is correspondingly enhanced bythe in situ doping. As a result, the rate of diffusion of phosphorusthrough the in situ-doped Ge is found to be significantly enhanced overthe rate of phosphorus diffusion through intrinsic Ge. This enhancedrate of diffusion overcomes the loss of dopant that generally occursduring a diffusion step due to out-diffusion at the surface andinterfaces, and due to loss of dopant by interface defect trapping.Thus, by exploiting the diffusivity enhancement that results from the insitu doping step, the ex situ doping step is correspondingly enhanced.As a result, it is discovered that a level of doping concentrationheretofore unachievable can be attained by the two-step in situ-ex situdoping method. With this achievement, the resulting highly-dopedgermanium can be incorporated with silicon electronics to producemonolithic optoelectronic systems operating at a wavelength of 1550 nm.

In one example for applying this two step doping method to Ge, to attainan active device layer with a doping level higher than that achievableby conventional in situ doping during growth of a Ge layer, there isgrown on a selected substrate a layer of Ge for which a high carrierconcentration is desired. Referring to FIG. 3A, the substrate 12 can beprovided as any suitable material; for many applications it can bepreferred to select a material with a lattice structure that is similarto Ge or Si and that imposes a selected degree of mechanical strain onGe, to adjust the conduction band valleys in the manner described above.A suitable II-VI or III-V substrate can be employed for manyapplications. Silicon or quartz can be a preferred substrate materialfor many applications. The substrate can be doped with a suitable dopantspecies and concentration as-required for a given device application, asexplained in detail below.

In the conventional manner, the growth can be carried out on a baresubstrate or on a substrate including, e.g., a patterned layer thatlimits the extent of Ge growth for forming Ge mesas, as explained indetail below. For example, there can be provided a patterned layer ofsilicon dioxide including windows through which Ge growth can beconducted to laterally restrict the Ge growth. Such patterned growth canbe tailored for a given application, as described in detail below.

Referring to FIG. 3B, for many applications, it can be preferred to forma first Ge buffer layer 40, or strain layer, on the substrate 12 priorto formation of an active Ge layer. The buffer layer relaxes the straininduced by lattice mismatch with the substrate to a reasonable level,and further serves as a sink for dislocations. The buffer layer can beprovided as any suitable thickness, e.g., between about 15 nm and about80 nm, and can be doped or undoped, as-required for a given application.Under some processing conditions, the buffer layer can preferably beprovided with a thickness between about 25 nm and about 80 nm. An activeGe layer can then be formed on the buffer layer with any suitablethickness for a given application.

Any suitable growth process can be employed, including ALD, MBE, andCVD, such as ultra-high vacuum CVD (UHVCVD). An UHVCVD process will bedescribed here by way of example, but such is not limiting.

In one such example process, in which Ge is formed on a siliconsubstrate, UHVCVD is conducted, at pressures less than about 1×10⁻⁹torr, to produce the Ge layer or layers on the substrate. The depositionchamber temperature is first set to, e.g., about 720° C., with hydrogenflowing to remove from the silicon surface any native oxide afterconventional cleaning of the substrate, to further clean the siliconsurface, and to passivate the substrate surface area of growth. Thehydrogen flow rate can be provided as, e.g., about 5.5 sccm, giving achamber pressure of about 3.4×10⁻⁴ mbar.

After a selected time for cleaning, e.g., about 20 min, the systemtemperature is decreased to a temperature for growth of the Ge bufferlayer, e.g., a temperature between about 320° C. and about 500° C. Oncestable at the selected buffer layer growth temperature, a germaniumprecursor gas, such as GeH₄, is delivered to substrate. For example, abuffer layer growth temperature of about 360° C. can be employed with aGeH₄ flow rate of about 7.5 sccm for about 60 min, with a chamberpressure of about 8×10⁻⁴ mbar, to produce a buffer layer.

It is recognized that the Ge buffer layer is not required in general andcan be excluded from the Ge layer growth for suitable applications. Forexample, if a selected Ge growth process provides reduced strain, thenthe buffer layer, which includes dopant traps at which dopant atoms maysegregate, can be eliminated. This can be an attractive technique wherea very high doping level is desired. As a result, for a given processsequence and device application, if such a buffer layer is not desired,then this process step can be omitted.

With the buffer layer in place on the silicon substrate, an active Gelayer 42 is then formed, referring to FIG. 3C. Maintaining the substratein situ, the deposition chamber temperature is raised to a selectedactive layer deposition temperature, e.g., between about 500° C. andabout 720° C., e.g., about 650° C. This Ge layer growth is conducteduntil a selected Ge active layer thickness is achieved, e.g., for about120 min. During growth, the Ge precursor gas GeH₄ is flowed at aselected flow rate, e.g., about 3.5 sccm and if desired, a selecteddopant gas is introduced to dope the Ge layer in situ during growth. Forexample, a gas of PH₃ can be introduced at a rate of, e.g., about 12sccm, with a chamber pressure of 4×10⁻⁴ mbar, to incorporate phosphorusatoms into the growing Ge layer in the manner described above. Thedopant flow rate is selected based on the corresponding level of dopantincorporation that can be accommodated by the Ge layer and based on thedilution of the dopant the gas phase.

To achieve a high in situ doping level, it can be preferred for manyapplications to employ a ratio of Ge precursor gas flow and dopantprecursor gas flow of between about 3:1 and about 4:1. For manyapplications it can be desired to produce with the in situ doping step adopant concentration of at least about 5×10¹⁸ cm⁻³, and more preferablyto produce an in situ doping concentration of at least about 1×10¹⁹ at areasonable Ge growth temperature, e.g., 650° C. Achievement of a higherin situ doping concentration is limited by a balance between Pout-diffusion and Ge crystal quality. Ge crystallinity degrades atgrowth temperatures below about 600° C. while P significantlyout-diffuses at temperatures about 650° C. Due to the small growthwindow that is thusly defined, the achievable in situ P dopingconcentration is limited. UHVCVD growth at a temperature of about 650°C. can therefore be preferred for doping with P for many applications.

With the active Ge layer and any underlying buffer layer produced, thefirst step of the doping process, namely, the in situ step, is complete.The resulting doping concentration of the active Ge layer is termed thebackground in situ doping level, and is the concentration that will beenhanced by the second, ex situ, doping step.

Referring again to FIG. 1, the ex situ doping process 23 can becompleted in any suitable manner for adding to the active layer thedopant to be diffused into the active layer. No particular technique isrequired for providing a dopant reservoir to be diffused into the activelayer. In one example, as shown in FIG. 1, there can be provided a soliddiffusion source 29 on top of the Ge active layer. In the Ge examplegiven here, a reservoir of phosphorus atoms can be provided on the Geactive layer surface for diffusion into the layer. But any suitableprocess can be employed for forming the solid diffusion source and asexplained above, ion implantation or other suitable process canalternatively be employed to provide a source of dopant at the Ge activelayer, as described in detail below.

Considering first the formation of a solid diffusion source, or dopantreservoir, formed on the active layer, phosphorus atoms can be providedby CVD, ALD, MBE, or other suitable process, to form several monolayers,a single monolayer, or a fraction of a monolayer of phosphorus dopantatom coverage on the Ge active layer surface, producing a so-calleddelta doping layer.

In one example of this step, immediately after formation of the Geactive layer and buffer layer by UHVCVD, the CVD chamber temperature isdecreased from the Ge active layer growth temperature to a temperatureof between, e.g., about 360° C. and about 450° C., e.g., about 400° C. A5.5 sccm flow rate of H₂ is continued for a selected duration tostabilize the temperature and then a phosphorus precursor, e.g., PH₃ isintroduced at a flow rate of, e.g., about 12 sccm for a durationsuitable to provide a selected coverage of phosphorus atoms, e.g.,between about 5-10 min.

The flow rate is preferably selected to achieve a high level of dopantatoms on the Ge layer surface. Referring to FIG. 3D, this results in adopant layer 44 on top of the active Ge layer 42. The dopant layer 44can be a monolayer, partial monolayer, or multiple layers of atoms.

Once the layer 44 of phosphorus atoms is produced, the layer isimmediately capped with an encapsulating layer of Ge 46, as shown inFIG. 3E. The function of the encapsulating layer is to maintain thelayer of delta doping phosphorus atoms in place and if desired, toprovide a surface for the deposition of additional phosphorus atoms. TheGe encapsulating layer can be any suitable thickness, e.g., betweenabout 1 nm and about 20 nm as a minimum thickness, and can be providedas a fraction of a monolayer, a monolayer, or several monolayers of Ge.The Ge encapsulation layer can be doped or undoped.

The Ge layer can be formed by continuing the CVD sequence by, e.g.,flowing GeH₄ at a flow rate of, e.g., about 3 sccm for a selectedduration, e.g., 10 min. The temperature can be adjusted or can bemaintained at the dopant deposition temperature. For many applicationsit can be preferred to maintain the temperature at the dopant depositiontemperature. It is preferred that the phosphorus dopant and Geencapsulation layers be deposited at relatively low temperatures atwhich the solid solubility of the Ge and P is increased.

Referring back to FIG. 1, subsequent to dopant reservoir formation, thedopant is caused to diffuse 30 into the active Ge layer to enhance thein situ doping level. Therefore, more than one delta doping layer can beincluded, if desired, to provide sufficient dopant atoms for achieving aselected doping level of the Ge active layer by the subsequent diffusionstep. Thus, any suitable number of dopant and encapsulating layers canbe included, e.g., at least about three, and even eight or more. FIG. 3Fillustrates an example of this condition, with four doping layers 44provided, separated by encapsulation layers of Ge 46.

FIG. 4 is a plot of an example gas flow control scenario for conductingthis Ge layer growth and delta doping layer formation all within asingle CVD sequence. At a first buffer layer growth temperature, e.g.,360° C., GeH₄ is flowed to produce an undoped Ge buffer layer, then at aselected active layer growth temperature, e.g., 650° C., GeH₄ and PH₃are flowed to form a Ge active layer that is in situ doped withphosphorus. Thereafter, an alternating sequence of GeH₄ and PH₃ areflowed to form delta doping phosphorus layers separated by Geencapsulation layers. This example process can be modified to dope theGe encapsulation layers by maintaining the flow of GeH₄ both duringdelta doping layer formation and during encapsulation layer formation.

Once a selected number of phosphorus doping layers and encapsulatinglayers are provided, a thicker germanium layer 47, shown in FIG. 3F, canbe provided as a capping layer, by, e.g., a Ge growth step of about20-30 min in duration in the manner described above. This capping layerprotects the surface from oxidation and contamination, and furtherprevents out-diffusion of dopant from the doping layers. An oxide,nitride, silicon, or other material capping layer can alternatively beemployed to preserve the delta doping stack surface.

At this point, a thermal annealing process or other suitable method iscarried out to cause the dopant atoms in the stack of delta dopantlayers to diffuse into the germanium active layer to increase the activedopant concentration in the active layer. For many applications, athermal annealing process can be preferred for its ability to repaircrystal lattice damage that may exist in the active layer. To enablesufficient diffusion of dopant from the delta doping layers, anannealing temperature of at least about 500° C. can be preferred, and amaximum annealing temperature that is below the melting point of Ge,e.g., about 800° C., can be preferred.

For many applications, a rapid thermal anneal (RTA) process conducted ata temperature within this temperature range can be employed, for aduration of, e.g., between about 5 sec and about 5 min. For example, anRTA step for 3 min at 600° C. can be suitable, while an RTA step for 30s at 700° C. can also be suitable. It is therefore to be recognized thatempirical analysis is generally required to optimize the duration andtemperature of the diffusion step. This step results in the uniformdoping of the germanium active layer with the activated phosphorus atomsat a concentration above that produced by the in situ doping step. Thestack of encapsulated dopant layers is thereby found to provide acapsule of solid source dopant that diffuses through the entire systemduring annealing to provide a high active dopant concentrationthroughout the active layer.

Referring to FIG. 3G, after diffusion of dopant from the delta dopinglayers into the Ge active layer, there can be conducted achemo-mechanical polishing step (CMP) to remove the delta doped layersand the intermediate Ge encapsulation layers from the surface 55 of theGe active layer. This results in an active layer 58 having the desiredhigh-concentration activated dopant with all dopant source removed. Withthe completion of the CMP step, high-concentration activated doping ofthe Ge active layer is complete.

This example of a UHVCVD sequence for the in situ and ex situ dopingprocess is not meant to be limiting; as explained above, other processescan be employed. For example, Ge buffer and active layers can be grownand in situ-doped by a CVD process, and then each delta doping layerformed by a suitable ALD or MBE process. Alternatively, Ge active layergrowth and in situ doping, as well as delta doping layer formation, canbe carried out all with MBE processes, or all with ALD processes. Formany applications, a CVD process is particularly convenient because ofits compatibility with conventional silicon CMOS processing. Integrationwith a conventional CMOS fabrication sequence and fabrication facilityis correspondingly most convenient with a CVD process.

As explained above, for any process to be employed for the in situdoping step, any suitable ex situ doping process can be employed. It isnot required that the ex situ doping be conducted by the same process asthe in situ doping, and can be conducted with an entirely differentprocess. In addition to the CVD, MBE, and ALD processes described above,in a further example of an ex situ doping step, the dopant species canbe ion-implanted into the active Ge layer for subsequent diffusion intothe bulk of the Ge layer.

Referring to FIG. 5A, in one example of this process, after in situdoping of a Ge layer during growth in the manner described above, dopantatoms are implanted 50 into the surface of the Ge active layer 42. Oneor more dopant species can be implanted into the Ge active layer. Forexample, phosphorus ions, arsenic (As) ions, and antimony (Sb) ions, orother suitable dopant ions, can be implanted alone or in combinationinto the surface of the Ge active layer. Suitable implantationparameters include an implantation energy between about 100 keV andabout 400 keV, a dose of between about 3×10¹⁵ cm⁻² and about 7×10¹⁵cm⁻², and a suitable tilt angle, e.g., about 7°.

As shown in FIG. 5A, the ion implantation process causes a layer ofdamage 52 at the surface of the active Ge layer 42, extending into thedepth of the active Ge layer that corresponds to the energy and dose ofthe implantation. This damaged layer includes a high concentration ofthe implanted dopant and operates as a solid diffusion source, orreservoir of dopant, that can be diffused into the depth of the Geactive layer.

Following the ion implantation step, the structure is thermallyannealed, in the manner described above, or subjected to an alternativesuitable process, to diffuse the implanted dopant into the Ge activelayer. RTA processing at a temperature between about 600° C. and about800° C. for a duration of between about 30 s and about 1880 s can bepreferably employed for many applications.

Referring to FIG. 5B, after diffusion of the implanted ions into the Geactive layer, there is conducted a chemo-mechanical polishing step (CMP)to remove the damaged region 52 from the surface of the Ge active layer.This results in a reduced-thickness active layer 55 having a polishedsurface 58 at which the crystal lattice damage from the ion implantationprocess is removed. With the completion of the CMP step,high-concentration activated doping of the Ge active layer is complete.

The ex situ doping step can be conducted with processes other than thosedescribed above and is not limited to ion implantation or delta dopantlayer formation. Vapor-source diffusion, liquid-source diffusion, orother diffusion process can be employed to ex situ dope a semiconductinglayer after in situ growth and doping of the layer.

With the high active dopant concentration enabled by the two-step insitu-ex situ doping process, optoelectronic devices can be fabricated toachieve operation that is enabled by the doping. Continuing with theexample of high-concentration Ge doping, there can be produced Ge lasersfor operation in a silicon-based optoelectronic system employing such Gelasers.

It has been estimated that an n-type doping level of 1×10¹⁹ cm⁻³ shouldyield a gain in a Ge Fabry-Perot cavity of about 50 cm⁻¹. This level oflevel gain can lead to lasing when pumped optically because opticallosses are mainly limited to facet losses and free carrier losses in Ge.Conversely, for electrical pumping of such a cavity, additional lossesdue to the electrical contacts, free carrier losses in system materialssuch as doped polycrystalline Si, and losses due to interaction with thecontact metal, must be overcome to enable lasing. Modeling of modepropagation in Ge waveguides with electrical contacts shows that theseadditional losses are >100 cm⁻¹.

To overcome these system losses, the Ge gain must be correspondinglyincreased, and such is attained by increasing the n-type dopingconcentration to a level of about 3-5×10¹⁹ cm⁻³. The in situ-ex situdoping process described above can achieve n-type doping levels of>4×10¹⁹ cm⁻³ to meet this requirement. By correlation ofphotoluminescence (PL) intensity, n-type doping level, and measuredmaterial gain, it is determined herein that an n-type doping level of4×10¹⁹ cm⁻³ corresponds to a Ge material gain of >400 cm⁻¹, which issufficient to overcome the losses in an electrically pumped laserdevice, and thereby that enables a silicon-based Ge opto-electronicsystem.

FIG. 6A is a perspective schematic view of an example of the main bodyof a vertical-injection Ge laser 60 including a Ge laser cavity 62having a high-concentration active doping level that enables lasing withelectrical pumping. Referring to FIG. 7A, in one example silicon-basedprocess for fabricating the vertical-injection Ge laser body, there isprovided a silicon substrate 70 having a selected doping level, e.g.,about 1×10¹⁹ cm⁻³, that is sufficiently high for the substrate tooperate as a contact electrode to the Ge laser cavity. The siliconsubstrate is oxidized, e.g., by thermal oxidation, to form a layer ofsilicon dioxide (SiO₂) 72 on both sides of the substrate, with an oxidethickness of e.g., between about 300 nm and about 500 nm.

Referring to FIGS. 7B-7C, in a photolithographic step the top layer ofsilicon dioxide 70 is patterned and etched to define a trench 74 in theoxide layer, maintaining some thickness 76 of oxide at the bottom of thetrench. This trench definition etch can be conducted by reactive ionetching (RIE) or other suitable process that preferably maintainsvertical trench sidewalls in the oxide thickness. Then as shown in FIG.7C a buffered oxide etch (BOE) wet etch can be employed to remove theoxide thickness 76 from the bottom of the trench while avoiding plasmadamage to the surface of the silicon substrate at the location for Gegrowth. A fully exposed window 78 in the oxide layer is thereby formed.

Referring to FIG. 7D, the in situ-ex situ doping process described aboveis then conducted with the growth of the Ge laser cavity material. Inone example, there is grown on the silicon substrate in the oxide windowa Ge buffer layer 80 of between about 20 nm and about 100 nm inthickness, an n+ phosphorus-doped Ge active layer 82 of between about300 nm and about 500 nm in thickness, and a stack 84 of phosphorus deltadoping layers and Ge encapsulation layers also of between about 300 nmand about 500 nm in thickness, in the manner described above. Then asshown in FIG. 7E, there is formed by, e.g., plasma enhanced chemicalvapor deposition (PECVD) a capping layer of silicon dioxide, of betweenabout 300 nm and about 500 nm in thickness, or other suitable material,to protect the Ge and dopant layers. A thermal annealing step in themanner described above is then conducted to cause diffusion of dopantfrom the delta doping layers into the Ge active layer.

Alternatively, after growth and in situ doping of the active layer,there can be implanted into the Ge active layer 82 a dose of dopant ionssufficient for doping the active layer by diffusion from the implantedregion at the surface, in the manner described above. A thermalannealing step is then conducted in the manner described above to causediffusion of the implanted dopant ions from the surface of the Ge activelayer into the bulk of the layer.

Whatever ex situ doping process is employed, at the completion of theannealing step or during a subsequent step, CMP or other planarizingprocess is employed in the manner described above to remove the deltadoping and encapsulation layers, or to remove the ion-implanted damagesurface region, to thereby expose the surface 88 of the Ge cavity activelayer.

Referring to FIG. 7G, next there is formed the top contact to the Gecavity. In one example process for such, there is deposited on the topsurface of the structure a layer 90 of amorphous Si of about 200 nm inthickness. A low-temperature, long-duration annealing step is preferablyemployed to drive out H₂. In one example long-duration anneal, there isconducted a first, twelve-hour anneal at about 150° C.-200° C., asecond, eight-hour anneal at about 200° C., a third, two-hour anneal atabout 300° C., a fourth, two-hour anneal at 350° C., and a final,one-hour anneal at 400° C. Then the amorphous silicon is doped by, e.g.,boron implantation, to form a p-doped layer of sufficient doping tooperate as a contact electrode to the Ge laser cavity. At this point,the backside silicon dioxide layer is removed. Then using a suitable RTAprocess, e.g., at about 750° C. for about 1 minute, the implanted dopantis activated and the amorphous silicon is crystallized topolycrystalline silicon.

As shown in FIG. 7H, photolithography is conducted to pattern a layer ofphotoresist, and the p-doped polysilicon layer 90 is etched, e.g., byRIE, down to the underlying oxide layer 72, to define the upperelectrode. Then as shown in FIG. 7I, photolithography is conducted topattern a layer of photoresist, and the underlying oxide layer 72 isetched, e.g., by RIE, down to the silicon substrate 70 to define awindow for making metal contact to the substrate, which operates as thelower electrode.

To complete the laser body structure, as shown in FIG. 7J, there isdeposited a layer of a suitable metal for the metal contacts, e.g., ametal stack of Ti/TiN/Al, having corresponding thicknesses of about 100nm, about 1 μm, and about 100 nm, respectively. As shown in FIG. 7K, themetal stack is lithographically patterned, e.g., by RIE, to define a topmetal contact 96 to the upper p-type polysilicon layer and the bottommetal contact 98 to the lower n-type silicon substrate. With this step,the body of the Ge laser cavity and electrical connection to the cavity,is complete for a vertical-injection Ge laser. This fabrication sequencecan be seamlessly integrated into a conventional CMOS fabricationprocess, and therefore processing to form corresponding Si electronicsand other features and devices in the optoelectronic system can beconducted immediately following this process.

Electrically-pumped Ge laser cavity structures like that just describedrequire both high doping concentration and a relatively thick,defect-free laser cavity material for successful operation of the lasersystem. The doping and laser fabrication processes described aboveprovide both of these requirements, which have historically beendifficult to achieve. The uniformity of the high-concentration dopingthat is provided through the thickness of the Ge active layer isparticularly important for enabling laser fabrication. The ability toachieve this condition with use of CMOS-compatible fabrication processesand equipment enables integration of these processes and the resultingdevices with silicon-based CMOS fabrication sequences in high-volumeprocessing operations.

Example 1 Ge In Situ Doping and Ex Situ Doping with Delta Dopant Layers

Ge layers were epitaxially grown on 6″ Si (100) substrates using ahot-wall UHVCVD reactor. A 30 nm-thick Ge buffer layer was firstdirectly grown on the Si substrates at a temperature of 360° C. Then a300 nm-thick Ge layer with in situ doping of phosphorous at a dopinglevel of 1×10¹⁹ cm⁻³ was grown at an elevated temperature of 650° C.with gas flow of 3.8 sccm of GeH₄ and 12 sccm PH₃. For comparison, therewas also grown a 300 nm-thick Ge layer that was not in situ-doped duringthe Ge growth.

The substrates were then exposed to PH₃ gas flow of 12 sccm at 400° C.for 5 min to deposit a layer of P atoms onto the Ge surface while at thesame time desorbing hydrogen. Subsequently, a 60 nm-thick intrinsic Gelayer was deposited at 400° C. Several cycles of PH₃ saturation andintrinsic Ge growth were performed in the reactor to encapsulatemultiple layers of P dopant atoms. For several samples, fourencapsulated dopant layers were formed, while for several other samples,eight encapsulated dopant layers were formed. A layer of 100 nm-thickSiO₂ was then deposited on the top Ge encapsulation layer as a cappinglayer to prevent out-diffusion during a subsequent annealing. The layerswere then annealed by RTA at temperatures ranging from 600° C. to 750°C. and a range of annealing times.

Secondary ion mass spectroscopy (SIMS) measurements were performed onthe as-grown and annealed samples to determine the diffusion profiles ofthe P dopant before and after the annealing steps. The P dopant depthprofiles were recorded as a function of depth with an accuracy of 2 nm.

FIG. 8 is a plot of measured P dopant concentration as a function ofdepth through the delta doping layers and the Ge active layer just afterdelta doping layer formation, prior to annealing. In the bulk of the Gelayer thickness, a phosphorus doping concentration of about 6×10¹⁸ cm⁻³was measured. The P peak concentration at ˜335 nm depth shows theaccumulation of phosphorus in the region of the P-doped encapsulationlayers. During the deposition of the intrinsic Ge encapsulation layersbetween the delta layers, the reactor temperature is kept under 400° C.but P diffusion occurs. Other P concentration peaks within the depth of335 nm are not very distinct due to asymmetrical P diffusion. The dopantpeak is observed to decay into in situ-doped Ge layer with a decaylength of 100 nm. The high P dopant concentration at the interface of Gebuffer layer and Si substrate is due to dopant accumulation in theundoped Ge buffer layer, which acts as a dopant trap due to a highdislocation density.

Hall Effect measurements were performed on the as-grown Ge layers todetermine the degree of electrically-activated P dopants. The measuredactive carrier concentration was 1.5×10¹⁹ cm⁻³, compared to the averagephysical concentration of 4.4×10¹⁹ cm⁻³ determined from the SIMS profileas the integrated P concentration over the depth, including the deltalayers, divided by the thickness. This discrepancy in measurementconfirms that the P atoms of the delta layers are not electricallyactivated in Ge layer and that annealing of the structures is preferredto cause P diffusion into the in situ-doped Ge layer to achieve higherdoping concentration with good single crystalline quality.

Rapid thermal annealing, under various conditions, was performed on thestructures, and SIMS and Hall effect measurements were used to determinethe dopant distribution profiles and the electrically activated dopantconcentrations. The standard measurement error for the Hall effectmeasurement setup was ±10%.

FIG. 9 is a plot of phosphorus dopant concentration as a function ofdepth. For clarity, in this plot the location of the interface betweenthe bottom-most delta doping layer and the surface of the Ge activelayer is set to zero depth. Driven by the concentration gradient, the Pdopant was found to diffuse deeper into Ge active layer from the deltalayers with longer annealing times. The annealing times were judged inview of a desire for high carrier concentration and uniform dopantdistribution profile through the thickness of the Ge active layer. Basedon these considerations, it was concluded that an RTA at 600° C. for 3min obtained the optimal dopant diffusion for the process parametersemployed here. In this case, an evenly distributed average carrierconcentration of 2.5×10¹⁹ cm⁻³ was achieved in the single crystalline Geactive layer as measured by SIMS. The Hall Effect measurement data forthe 3 min RTA step at 600° C. indicated an activated carrierconcentration of (2.8±0.3)×10¹⁹ cm⁻³ in the bulk of the Ge active layer.This demonstrates that the phosphorus dopant is completely activated bya 3 min RTA step at 600° C.

Based on the data from RTA processes at 600° C., 650° C., and 700° C.,the doubly-charged intrinsic diffusion coefficient for Ge was determinedand is plotted in FIG. 10. Generally, diffusivity is related totemperature by an Arrhenius characteristic and hence, the expressionD=D₀*e^(−Ea/kT) was employed to fit the three data points. The extractedactivation energy, E_(a)=1.98 eV and the pre-exponential coefficient isD₀*=2.2×10⁻⁴ cm²/s, which is in the order of the predicted calculations.The intrinsic carrier concentration in tensile-strained Ge is 2.09×10¹⁷cm⁻³ at 600° C. and 3.88×10¹⁷ cm⁻³ at 700° C. Due to the in situ dopingstep during the Ge active layer growth, carrier concentration in the Gelayer before annealing was increased to 7×10¹⁸ cm⁻³. Therefore, theextrinsic diffusivities in the in-situ doped Ge region, shown as thesquares in FIG. 10, are about 2 orders higher than the diffusivities inintrinsic Ge.

These results confirm that the quadratic dependence of diffusivity oncarrier concentration for Ge causes the P dopant diffusion to besignificantly enhanced by in situ doping. Compared to the intrinsicepitaxial Ge layer grown on Si substrate, diffusion in in situ dopedGe-on-Si film is significantly faster. During annealing, dopant loss isalso observed, which can be concluded from the decrease of average Pconcentration with longer annealing times. Therefore, faster diffusioninto the epitaxial Ge layer is preferable to compete with the dopantout-diffusion to the surface and interface.

The enhanced diffusivity resulting from in situ diffusion was furtherconfirmed by comparison of SIMS dopant profile data for in situ-doped Geactive layers and for undoped Ge active layers. FIG. 11 is a plot ofphosphorus dopant concentration as a function of depth for the Ge activelayer that was grown without in situ doping during growth. In the plot,the interface between the bottom-most delta doping layer and surface ofthe Ge active layer is at a depth of about 275 nm. The plot shows boththe dopant depth profile after delta doping layer formation and afteranneal subsequent to delta doping layer formation. In the bulk of the Geactive layer, the dopant concentration is about 3×10¹⁷ cm⁻³. After anRTA step of 1 min at 600° C., the dopant concentration in the bulk ofthe Ge active layer is about 3×10¹⁸ cm⁻³.

This measurement dramatically compares with the measurement of(2.8±0.3)×10¹⁹ cm⁻³ that was taken for the phosphorus concentration inthe bulk of the Ge active layer after in situ doping and annealing afterformation of the delta dopant stack. The combination of the in situdoping step with the ex situ doping step is demonstrated to result in atleast one order of magnitude increase in dopant concentration in thebulk of the Ge active layer.

To further quantify the activated dopant concentration, there wasmeasured the photoluminescence of the Ge active layer samples under thevarious annealing conditions. The optically-active dopant concentrationis equivalent to the electrically-active concentration and thismeasurement therefore can provide further evidence of activated dopantconcentration. FIG. 12 is a plot of measured photoluminescence intensityas a function of wavelength for annealing at various times andtemperatures, as well as for a sample for which no anneal was conducted,and for a Ge active layer, referred to as epi-Ge, which was in situdoped but for which no ex situ doping was conducted. The shift in thepeak to longer wavelengths is consistent with the increased carrierconcentration.

Example 2 Ge In Situ Doping and Ex Situ Doping by Ion Implantation

Ge layers were epitaxially grown on 6″ Si (100) substrates using ahot-wall UHVCVD reactor. A 30 nm-thick Ge buffer layer was firstdirectly grown on the Si substrates at a temperature of 360° C. Then a500 nm-thick Ge layer with in situ-doped phosphorous at a doping levelof 1×10¹⁹ cm⁻³ was grown at an elevated temperature of 650° C. with a3.8 sccm GeH₄ gas flow and a 12 sccm PH₃ gas flow.

After active layer growth, three different ion implantation processeswere conducted, as given in Table I below.

TABLE 1 Tilt Energy Dose Energy Dose # angle ° ions (keV) (cm⁻²) ions(keV) (cm⁻²) 1 7 P 100 7.3 × 10¹⁵ — — — 2 7 P 100 3.5 × 10¹⁵ As 250 3.8× 10¹⁵ 3 7 P 100 3.5 × 10¹⁵ Sb 375 4.1 × 10¹⁵

After implantation, the samples were annealed to diffuse the implantedions into the Ge active layer. RTA processes having a temperature of600° C.-800° C., and durations of 30 s-180 s were conducted.

FIG. 13 is a plot of measured photoluminescence intensity as a functionof wavelength for the phosphorus-implanted Ge active layers. Themeasured peak intensity at a wavelength of 1660 nm corresponds to anactivated dopant concentration of about 4×10¹⁹ cm⁻³ that is produced bythe phosphorus implantation and annealing at 750° C. for 1 min.

FIG. 14 is a plot of photoluminescence intensity as a function ofwavelength for the phosphorus-arsenic co-ion implantation combinationinto Ge active layers. The measured peak intensity at a wavelength of1680 nm corresponds to an activated dopant concentration of about5.5×10¹⁹ cm⁻³ that is produced by the phosphorus implantation andannealing at 750° C. for 1 min.

FIG. 15 is a plot of photoluminescence intensity as a function ofwavelength for the phosphorus-antimony co-ion implantation combinationinto Ge active layers. The measured peak intensity at a wavelength of1700 nm corresponds to an activated dopant concentration of about8.7×10¹⁹ cm that is produced by the phosphorus implantation andannealing at 700° C. for 3 min.

Example 3 Ge Laser Fabrication and Operation

A vertical-injection, electrically-pumped Ge laser was fabricated in themanner of the fabrication sequence of FIGS. 7A-7K described above,employing the process of Example 1 above for producing a Ge buffer layerof 30 nm in thickness and Ge active layer of about 300 nm, doped in situwith phosphorus doping of 1×10¹⁹ cm⁻³, here conducted as mesa growth ina trench window in a silicon dioxide layer as in FIG. 7D. Fourencapsulated phosphorus delta layers were formed, with a silicon dioxidecapping layer of 100 nm in thickness to prevent out diffusion. Thermalannealing for diffusion of phosphorus from the delta doped layers intothe Ge active layer was conducted by RTA at 700° C. for 1 minute. Thenthe structure was then planarized, as in FIG. 7F, by CMP, to remove thedelta doping and capping layers from the active Ge layer surface. Theremaining thickness of the Ge active layer after CMP as measured acrossthe substrate and found to vary between 100 nm and 300 nm, depending onsubstrate location. Due to severe dishing of the waveguides after CMPthe supported optical modes in the waveguides could not be determinedexactly. Up to six cavity modes can be supported in the largestwaveguides.

In the manner of FIG. 7G-7K, a 180 nm-thick amorphous-Si layer was thendeposited by PECVD and subsequently phosphorus-implanted to a dopinglevel of 10²⁰ cm⁻³. After a dopant activation annealing step at 750° C.for 1 minute, a metal layer stack consisting of Ti and Al was depositedfor forming top and bottom contacts. The oxide trench was determined toprovide excellent electrical current confinement. In order to assureeven carrier injection into the n-type Ge active layer, the top contactmetal was deposited on top of the Ge layer. After dicing, the waveguideswere cleaved to expose the Ge active layer waveguide facets. A thinoxide layer was deposited on the facets to protect against contaminationand catastrophic optical mirror damage which was observed in devicesthat did not have oxide protection.

Emission from the Ge waveguide cavity was measured using a Horiba MicroPL system equipped with a cooled InGaAs detector with lock-in detection.The emission power measurement was calibrated using light from acommercial 1550 nm laser that was coupled into a single mode opticalfiber with the fiber end at the sample location. In calibration it wasverified that the detection was linear with input power. Electricalpumping of the Ge cavity was supplied by a pulse generator with currentpulse widths in the range of 20 μs to 100 ms. The duty cycle was variedbetween 2% and 50%, typically 4%, to reduce electrical current heatingeffects. The laser was contacted with metal probes and the current wasmeasured using an inductive sensor placed directly in the biasingcircuit. FIG. 16 illustrates the experimental setup 150 forcharacterization of the Ge laser 60.

The Ge laser cavity was electrically pumped and the resulting emissionspectrum measured. All measurements were performed with the samplemounted on a thermo-electric cooler at 15° C. The local devicetemperature was likely higher, however, due to the high currentinjection, but could not be reliably determined.

FIG. 17A is a plot of the measured emission spectrum below the lasingthreshold. FIG. 17B is a plot of the measured emission spectrum abovethe lasing threshold. The spectra employed short integration times toassure wide spectrum analyses. The plot of FIG. 17A demonstrates that nospectral features above the noise floor were emitted by the laser belowthe lasing threshold. When the injection current density was increasedabove threshold, sharp laser lines appeared, as shown in the plot ofFIG. 17B. The observed linewidth of the individual lines is below 1.2nm, which is the spectral resolution of the measurement set-up. Thelasing spectrum intensity plot of FIG. 17B shows two lines. The estimateof the cavity free spectral range (FSR) is 1 nm, and the line spacing inthe plot, 3 nm, is a possible multiple of the FSR.

FIG. 18 is a plot of the laser emission power as a function of currentdensity. The lasing threshold at about 280 kA/cm² is clearly visible.This measurement was taken using the measurement system of FIG. 16 witha wide instrumental spectral resolution of 10 nm. The number of datapoints was limited by metal contact breakdown at high current levels.The optical emission power of 1 mW is a lower estimate and was measuredfor a wavelength range of 1500 nm-1650 nm.

These examples demonstrate that the two-step in situ-ex situhigh-concentration doping method can produce uniform, activated n-typedoping in Ge as a laser gain medium, at a doping concentration that issufficient to produce in the gain medium a guided mode that overcomesthe losses in an electrically-pumped laser configuration of the gainmaterial, to enable the integration of a Ge laser gain medium into asilicon-based electro-optical system.

As described above, the two-step in situ-ex situ doping method is notlimited to doping of Ge; such is provided as an exemplary example. Anysemiconducting material for which electrical doping is desired can beprocessed in accordance with the two-step in situ-ex situ doping method.The method enables the production of devices and systems nothistorically attainable by conventional doping techniques.

It is recognized, of course, that those skilled in the art may makevarious modifications and additions to the processes of the inventionwithout departing from the spirit and scope of the present contributionto the art. Accordingly, it is to be understood that the protectionsought to be afforded hereby should be deemed to extend to the subjectmatter of the claims and all equivalents thereof fairly within the scopeof the invention.

We claim:
 1. A method of forming a photonic device, the methodcomprising: forming a germanium active layer on a silicon substrate; insitu n-type doping the germanium active layer during formation of theactive layer; forming a dopant atom reservoir of n-type dopant atoms atthe germanium active layer after formation of the active layer; anddiffusing dopant atoms from the dopant atom reservoir of n-type dopantatoms through the germanium active layer.
 2. The method of claim 1wherein forming the germanium active layer comprises a process selectedfrom the group of chemical vapor deposition, molecular beam epitaxy, andatomic layer deposition; and wherein forming the dopant atom reservoirof n-type dopant atoms at the germanium active layer comprises a processselected from the group of chemical vapor deposition, molecular beamepitaxy, and atomic layer deposition.
 3. The method of claim 2 whereinforming the germanium active layer consists of chemical vapor depositionof the germanium active layer, and wherein forming the dopant atomreservoir of n-type dopant atoms at the germanium active layer consistsof chemical vapor deposition of the dopant atom reservoir.
 4. The methodof claim 3 wherein forming the germanium active layer comprises chemicalvapor deposition comprising flow of GeH₄ precursor gas and wherein insitu doping of the germanium active layer comprises flow of PH₃precursor gas.
 5. The method of claim 1 wherein forming the dopant atomreservoir of n-type dopant atoms comprises forming at least a partialmonolayer of dopant atoms on a surface of the germanium active layer. 6.The method of claim 5 further comprising forming an encapsulation layeron top of the at least partial monolayer of dopant atoms.
 7. The methodof claim 5 wherein the encapsulation layer comprises a layer ofgermanium.
 8. The method of claim 5 wherein forming at least a partialmonolayer of dopant atoms comprises cyclically forming at least apartial monolayer of dopant atoms and an encapsulation layer to producea stack of encapsulated dopant atom layers on a surface of the germaniumactive layer.
 9. The method of claim 1 wherein forming the dopant atomreservoir of n-type dopant atoms comprises ion implantation of dopantatoms into the semiconducting material layer.
 10. The method of claim 1further comprising, after diffusing dopant atoms through the germaniumactive layer, removing the dopant atom reservoir of n-type dopant atomsfrom the germanium active layer.
 11. A structure for forming a photonicdevice comprising: a silicon substrate; an active layer of germaniumdisposed on the silicon substrate, the germanium active layer includingan n-type dopant concentration of at least about 5×10¹⁸ cm⁻³; and astack of at least one dopant reservoir layer disposed on top of thegermanium active layer, each dopant reservoir layer consisting of aleast a partial monolayer of phosphorus dopant atoms, a germaniumencapsulation layer being disposed between each dopant reservoir layerto in the stack.
 12. The structure of claim 11 further comprising agermanium buffer layer disposed between the substrate and the germaniumactive layer.
 13. The structure of claim 11 further comprising a layerselected from the group of amorphous silicon and polycrystallinesilicon, disposed on top of the stack of dopant reservoir layers. 14.The structure of claim 11 wherein the silicon substrate comprises assilicon-on-insulator substrate.
 15. The structure of claim 11 whereinthe germanium active layer includes an electrically activated n-typedopant concentration of at least about 2×10¹⁹ cm⁻³.
 16. Anelectrically-pumped photonic device comprising: two silicon electrodes,each electrode characterized by an electrical loss factor thatcontributes to an electrical loss total for the photonic device; and anactive layer of germanium disposed between the two silicon electrodesfor electrical pumping of the active layer; wherein the germanium activelayer supports an electrically-pumped guided mode as a laser gain mediumwith an electrically-activated n-type electrical dopant concentrationthat is greater than a background dopant concentration characteristic ofthe active layer as-formed, to overcome the electrical loss total forthe photonic device.
 17. The device of claim 16 wherein the germaniumactive layer is a mesa disposed in a window of a silicon dioxide layer.18. The device of claim 16 wherein one of the silicon electrodescomprises polycrystalline silicon.
 19. The device of claim 16 whereinone of the silicon electrodes comprises a layer of silicon on asilicon-on-insulator substrate.
 20. The device of claim 16 wherein thegermanium active layer includes an electrically activated n-type dopantconcentration of at least about 2×10¹⁹ cm⁻³.
 21. A method of forming aphotonic device, the method comprising: forming a first siliconelectrode; forming a germanium active layer on the first siliconelectrode while including n-type dopant atoms in the germanium layer,during formation of the layer, to produce a background electrical dopantconcentration that is greater than an intrinsic dopant concentration ofgermanium; forming a second silicon electrode on a surface of thegermanium active layer; and electrically doping the formed germaniumactive layer with additional dopant for supporting anelectrically-pumped guided mode as a laser gain medium with anelectrically-activated n-type electrical dopant concentration that isgreater than the background dopant concentration to overcome electricallosses of the photonic device.
 22. The method of claim 21 wherein thegermanium active layer is formed by chemical vapor deposition.
 23. Themethod of claim 21 wherein forming a first silicon electrode comprisesproviding an electrically doped silicon substrate.
 24. The method ofclaim 21 wherein forming a second electrode comprises forming a layer ofamorphous silicon on the germanium active layer and converting theamorphous silicon to polycrystalline silicon.
 25. The method of claim 21wherein electrically doping the formed germanium active layer comprisesdoping the germanium active layer with an electrically activated n-typedopant concentration of at least about 2×10¹⁹ cm⁻³.